The mantra does occur in the chaitanyabhAgavata. Chaitnya said it in Adi 14.145-146, and this is obvious when you see the original Bengali text as well:
hare krsna hare krsna krsna krsna hare hare, hare rama hare rama rama rama hare hare. ei sloka nama bali laya mahamantra, solanama batrisaaksara ei tantra.
This consists of sixteen names of the ...
Lord Vishnu doesn't have a fixed form but assumes any form for the sake of Devotees to reciprocate love.
In whatever way people surrender unto me, I reciprocate with them
accordingly. Everyone follows my path, knowingly or unknowingly, O son
of Pritha.(BG 4.11)
Whatever form a devotee seeks to worship with faith, I
steady the faith of such a devotee in that ...
According to Nivaran Tantra:
निराकारो महाविष्णुः साकारोऽपि क्षणे क्षणे ।
यदा साकाररुपोऽसौ द्विभुजो मुरलीधर: ॥
Mahavishnu is 'Nirakar' (without form), sometimes he gives 'Sakar Darshan' (to see him with form). During the time when we see him, he is in form of Murilidhar with Two hands.
(From Sarvollāsatantram, 16th Ulhassa)
Also, are there any other similar instances (need not be based on the
same practice of Tulabharam) described in the scriptures, where a
Husband or Wife (or anyone of kith and kin type) makes such kind of
philanthropic benefaction based dharmic donation i.e dāna (दान) to
Tulapurusha is mentioned among one of the most effective sin cleansing measures ...